The oligomerization of olefins such as isobutylene using a sulfonic acid-type ion exchange resin catalyst is well-known in the art. For instance, U.S. Pat. No. 4,100,220 describes isobutylene oligomerization using a sulfonic acid resin catalyst and tertiary butanol selectivity enhancing modifier. In addition, U.S. Pat. No. 4,447,668 discloses isobutylene oligomerization using sulfonic acid resin catalyst A-15 with methyl t-butyl ether as solvent. Further, U.S. Pat. No. 5,877,372 describes the selective oligomerization of isobutylene using a sulfonic acid resin catalyst, tertiary butanol selectivity enhancing modifier and isooctane diluent. Lastly, U.S. Pat. No. 6,376,731 discloses the oligomerization of isobutylene in the presence of a C3–C4 alkane diluent to enhance oligomerization selectivity and tertiary butanol to promote selectivity to diisobutylene.
The diisobutylene product may be used as such or may be hydrogenated to isooctane as described in U.S. Pat. No. 5,877,372 and No. 6,376,731. Diisobutylene and isooctane are potential fuel blending compositions.
I have surprisingly found that the use of sulfonic acid-type ion exchange resin catalysts during the oligomerization step results in sulfur impurities in the diisobutylene product. Unfortunately, the presence of such sulfur impurities presents problems in regard to the use of the diisobutylene product. In the case of diisobutylene hydrogenation to form isooctane, the presence of sulfur impurities in diisobutylene results in deactivation of the hydrogenation catalyst. For diisobutylene product added directly to fuel streams, the presence of sulfur impurities is undesirable since combustion of fuel streams containing these impurities results in the release of sulfur oxides which are noxious, corrosive, and therefore, present a serious pollution problem. The United States Environmental Protection Agency has imposed increasingly more stringent requirements to reduce the amount of sulfur in fuel streams to mitigate this problem. It is desirable to develop a simplified procedure to produce diisobutylene containing a reduced amount of sulfur impurities.
In sum, new methods to produce diisobutylene by oligomerization of isobutylene over a sulfonic acid-type ion exchange resin catalyst are needed. Particularly needed are processes that result in lower sulfur impurities.